The present invention relates generally to star trackers, and more particularly, to a system for determining stars in a star tracker field of view.
Satellites and other spacecraft are in widespread use for various purposes including scientific research and communications. Many scientific and communications missions, however, cannot be accurately fulfilled without consistently monitoring and controlling the 3-axis attitude of the spacecraft. In many applications, the satellite must be positioned to direct communication signals in particular directions or to receive signals from specifically located sources. Without accurate control over spacecraft 3-axis attitude, the transmission of such signals is hindered and at times impossible.
Many modern spacecraft use star trackers for 3-axis attitude control. The systems for 3-axis attitude determination generally include one or more star trackers and a separate 3-axis orthogonal gyroscope. During normal operation, star trackers provide continuous attitude information and the 3-axis orthogonal gyroscope is needed to provide rate information and is constantly corrected by the star trackers.
To improve spacecraft attitude control performance, determining stars in a star tracker field of view (FOV) algorithms employ a carefully designed star catalog. The current known art in star selection for star trackers mainly concentrates on generating star catalogs with certain properties. Typically, one method of locating stars in a tracker FOV uses a standard binary search that is sorted by declination only. This method is not very efficient because it involves searching through hundreds of stars to find the stars that are located within the FOV.
Various other methods use multiple overlapping sub-catalogs that contain stars for a specific FOV in the sky. At any point in time, a star tracker""s FOV will reside in one or more of these sub-catalogs. Each star that is in a FOV sub-catalog is tested to determine whether it is in the tracker FOV.
These types of methods require large amounts of memory storage and throughput.
The disadvantages associated with these conventional star determination techniques have made it apparent that a new technique for determining stars within or near a star tracker field is needed. Preferably, the new technique would be able to accurately locate stars within a tracker FOV without requiring large amounts of memory storage and throughput. The new technique should also not require searching through hundreds of stars to find the stars that are located within the FOV.
It is, therefore, an object of the invention to provide an improved and reliable system for determining stars in a star tracker field of view. Another object of the invention is to provide a system that does not require large amounts of memory storage and throughput.
In one embodiment of the invention, an apparatus for determining star location includes a star tracker, a star catalog and a controller. The star tracker is used to sense the positions of stars and generate signals corresponding to the positions of the stars as seen in its field of view. The star catalog contains star location data that is stored using primary and multiple secondary arrays sorted by both declination (DEC) and right ascension (RA). The controller checks the star catalog and determines which stars to track. The controller does this determination by using an algorithm to sort the primary and secondary arrays to determine which stars are located in the star tracker field of view. The controller then commands the star tracker to track these stars and uses them to determine the spacecraft attitude.
The present invention achieves an improved system for determining stars in a star tracker field of view. The present invention is advantageous in that it does not require searching through hundreds of stars to find the stars that are located within the FOV.
Additional advantages and features of the present invention will become apparent from the description that follows, and may be realized by means of the instrumentalities and combinations particularly pointed out in the appended claims, taken in conjunction with the accompanying drawings.